Gas in an atmospheric layer (5)
I believe the answer is:
ozone
'gas in an atmospheric layer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ozone that I've seen before include "A gas; threatened layer in air" , "Sea air (found in layer?)" , "Bracing seaside air" , "Gas forming screen against ultraviolet radiation in the stratosphere" , "Gas forming protective layer" .)